In recent years, it has become more and more common to use cellular radio communication systems for automatic exchange of information between devices for performing many different tasks, such as opening and closing of valves in for example a sewage system, measuring of temperature or pressure, updating of map information for a GPS-system in a car and more. This kind of automated communication, without user interaction, is often referred to as machine-to-machine (M2M) communication. As more and more devices, such as laptops, digital cameras, cars, outdoor thermometers, indoor thermometers, electricity meters and so on, become connected, the number of connections in the radio communication systems will increase drastically.
In Third Generation Partnership Project Long Term Evolution (3GPP LTE), all scheduling assignments, grants and commands are issued to specific Radio Network Temporary Identifiers (RNTI). The RNTI is a number between 0 and 216. Different types of RNTIs exist, such as the Paging RNTI (P-RNTI), System Information RNTI (SI-RNTI), etc. For example, a communication device (or user equipment, “UE”) that is reading e.g. System Information is looking for the commands assigned to the SI-RNTI on the Physical Downlink Control Channel (PDCCH). RNTIs can either be common to several communication devices, or unique to one specific communication device.
Specifically, the Cell RNTI (C-RNTI) is used to address a specific communication device in a connected state, such as RRC CONNECTED state in case of an LTE system. A communication device in RRC CONNECTED state has established a connection to a cellular radio communication network. Therefore, the communication device in RRC CONNECTED state needs at least one C-RNTI that is unique among the C-RNTIs assigned to other communication devices in RRC CONNECTED state in the same cell. Multiple RNTIs may be allocated to a communication device at the same time, i.e. in parallel. For example, a Semi-Persistent Scheduling RNTI (SPS-RNTI) may be assigned to a communication device in addition to the aforementioned C-RNTI.
The RNTIs in current LTE network are signalled by 16 bits, meaning that 216=65 536 values are possible. However, in practice, it can be speculated that if allocations of RNTIs are very closely in the RNTI space (in terms of the Hamming distance), this would lead to a high probability of RNTI misdetection. If this is the case, it is possible that only a fraction of the current RNTI number space can be utilized in practice.
The following problems make the C-RNTI values limited:                all RRC connections need at least one RNTI, i.e. the C-RNTI,        only one connection can be identified with one C-RNTI, and        some connections may require multiple RNTIs.        
Furthermore, as explained above, not all RNTI values are available for C-RNTI use, but only a subset (albeit a large one) is actually allocated for C-RNTIs.
In the following example, scheduling of a communication device on PDCCH (Physical Downlink Control Channel) using a C-RNTI as specified by 3GPP LTE is described. PDCCH is used for carrying e.g. downlink scheduling assignments and uplink scheduling grants. The assignments and grants include detailed information of PDSCH/PUSCH (Physical Downlink Shared Channel/Physical Uplink Shared Channel) resource indication, transport format, hybrid-ARQ (Automatic Repeat reQuest) information etc. A Cyclic Redundancy check (CRC) is attached to PDCCH payload, where the RNTI is included in CRC calculation. Upon on reception of PDCCH, the communication device will check the CRC using its unique C-RNTI. If the CRC matches, the communication device may conclude that the message is intended to it.
Consider the following scenario. It is assumed that data becomes available for transmission in the communication device, but the communication device does not have UL resources to transmit the data, even when the communication device is in the RRC CONNECTED state. Thus, the communication device requests resources with a Scheduling Request (SR) from a radio network node, such as an eNB. Then, the SR initiates a Random Access (RA) procedure if Physical Uplink Control Channel (PUCCH) resources are not allocated for transmission of the SR. In a contention based RA, the communication device selects a random preamble to be transmitted in Random Access Channel (RACH). For this case, the RA procedure is as follow:                The communication device transmits a random preamble selected by it on RACH (as noted above).        The radio network node responds with a RA Response (RAR) for the same preamble as transmitted by the communication device. RAR message includes a Scheduling Grant (SG) for an uplink transmission.        The communication device now responds to the RAR with a scheduled message 3 (as known from 3GPP-terminology) including a C-RNTI thus identifying the communication device.        The radio network node then replies with a Contention Resolution message. If the Contention Resolution message includes the same C-RNTI as the communication device has transmitted in message 3, the communication device regards the Random Access Procedure as successful.        
The above described method is contention based, because two communication devices can request resources at the same time with the same preamble. In this case the radio network indicates by means of C-RNTI in Contention Resolution message which of the communication device succeeds with the random access.
The Discontinuous Reception (DRX) procedure, defined as a part of the LTE Medium Access Control (MAC), specifies time periods during which a communication device is obliged to monitor the PDCCH. In DRX, an active time is defined for this purpose. In time periods, specified as active time, the communication device is not allowed to go to a sleep state, which consumes less power. Active time is calculated based on specific DRX timers and cycles in such a way that the network and the communication device have a similar understanding of when it is possible to schedule the communication device.
Some devices may transmit so called keep-alive messages just to avoid loss of the connection to avoid switching between connected and idle state. From a communication device perspective, it is beneficial to be connected continuously since data may be transmitted and received fast when needed (no time is wasted on setting up a connection). Furthermore, the network settings and the traffic generated at the communication device may cause the communication device to always stay connected even if the connection is only needed for short periods at a time. Hence, a C-RNTI may be occupied for long periods of time even if the communication device does not transmit a lot of information.
As the number of connections, due to for example automated communication from communication devices in the radio networks increases, it is possible that the current number of usable RNTI values is not enough to cater for all the devices in the network simultaneously. An example where this may happen is a dense sensor network including a huge amount of temperature/pressure/humidity sensors. In addition, there may be user equipments, such as cellular phones, in the same cell as the sensors. These user equipments may also be connected and, hence, consume (or occupy) a C-RNTI. A solution according to prior art is such that each communication device, i.e. sensor or user equipment, has its own connection to the network. Thus, each communication device requires a C-RNTI that is unique in the cell.
Next, a numerical example of the RNTI limitation is described. Consider X devices, such as weather sensors located in a cell. Each communication device is in RRC CONNECTED state, thereby a unique C-RNTI for each communication device is required. Each communication device sends a measurement report once in every 10 seconds. The size of the measurement report is 1000 bits. Thus, the traffic load generated by each device is:r=1000 bit/10 s=100 bps.
The aggregate traffic generated by X devices is r*X. Assume that the cell throughput of a typical LTE cell is 10 Mbps. From the system capacity point of view, it is possible to haveX=10 Mbps/100 bps=100 000 devices in a cell.
Therefore, since the RNTI space is limited to 16 bits (which yields 65 536 possible RNTI values), a problem of prior art solutions is that the number of RNTIs does not suffice.
When the RNTI space is exhausted, the network needs to drop connections of some devices to allow for other devices to connect instead. Switching frequently between connected and idle state increases amount of signalling messages, overhead and also battery consumption. For small devices having only very limited battery, this is not desirable.